1. Field of the Invention
The present invention relates to an optical transceiver module for optical communication, and more particularly, to a casing structure of a transceiver for optical fiber transmission having a high-speed transmission rate of 10 Gbit/s.
2. Description of the Related Art
A transceiver module for optical fiber transmission (optical transceiver module) is required to achieve a downsizing and higher speed operation in accordance with a widespread use of broadband network in recent years. For the higher speed operation, an optical transceiver module having a bit rate of 10 Gbit/s is now widely used. (For example, there are used ones having 9.95 Gbit/s and 10.7 Gbit/s compliant with Synchronous Optical Network (SONET), and one having 10.3 Gbit/s compliant with 10 Gigabit Ethernet (“Ethernet” is a registered trademark).) For the downsizing, a reduction in casing capacity is promoted from an older generation 300-pin Multi Source Agreement (MSA) standard to XENPAK, X2, XFP, and SFP+ (which are MSA standards).
On the other hand, a transmission device, on which the optical transceiver module is mounted, is required to suppress an intensity of unnecessary electromagnetic waves generated in the transmission device to a statutory limit value or less. For example, in the United States, it is necessary to satisfy the limit value of 53.9 dB (μV/m) or less which is defined in FCC, Part 15, Subpart B (in a case of Class B, distance: 3 m, frequency range: 1 GHz to 40 GHz).
Due to switching noise and the like of an IC, which is built in the optical transceiver module and operates at 10 Gbit/s, the optical transceiver module generates unnecessary electromagnetic waves in many cases at the frequency of 10 GHz and at the frequency of 20 GHz which is the higher harmonics content thereof. Thus, a design technology for reducing radiation of the unnecessary electromagnetic waves to an exterior of the device is important for both the transmission device and the optical transceiver module.
Kazushige Oki, et al., “The Design Challenge with the Predictable Analysis of the Heat Dissipation and the Electro Magnetic Radiation for 10 Gbps Pluggable Optical Transceivers”, 2006 Electronic Components and Technology Conference, pp. 1567-1572 (2006) (hereinafter, referred to as “Non-patent Document”) describes a technology for reducing the unnecessary electromagnetic waves at 10 GHz in an X2 MSA optical transceiver module. According to this technology, the structure of a transmitter optical sub-assembly (TOSA) is made with ingenuity, and an exposed portion of the TOSA of the optical transceiver module is connected to a frame ground to be electrically separated from a signal ground of a CAN structure portion of the TOSA. As a result, the unnecessary electromagnetic waves which are radiated through a TOSA/optical-connector connecting portion toward a front surface of the transmission device are reduced.
Further, JP 2006-84683 A discloses a further detailed technology for the TOSA structure of the Non-patent Document.
U.S. Pat. No. 6,999,323 B1 and U.S. Pat. No. 7,068,522 B2 each disclose a casing design technology for reducing radiation of the unnecessary electromagnetic waves in an XFP MSA optical transceiver module. According to this technology, a short circuit part (which is formed by an upper casing column, a board through electrode, and a lower casing column) formed of a conductor is provided at a substantially intermediate position with respect to a casing opening of an edge connector of a printed circuit board, whereby an effective length of the opening is reduced and the unnecessary electromagnetic waves leaking from the opening of the edge connector are reduced.
In recent years, as a condition for the radiation of the unnecessary electromagnetic waves of the transmission device (customer side), on which the optical transceiver module is mounted, becomes severer, when the optical transceiver module is configured with use of the above-mentioned conventional technologies, there is a risk in that the specification for the radiation of the unnecessary electromagnetic waves, which is required for the transmission device, cannot be satisfied. Particularly, diversification of the transmission device on the customer side increases, and, in accordance with an increase in the number of the optical transceiver modules mounted to the same transmission device (multiple mounting) and a reduction in shield amount of the transmission device as a result of cost-reduction of a shielding countermeasure for the transmission device, each of the optical transceiver modules is required to further reduce the radiation of the unnecessary electromagnetic waves.
In the course of examining the reduction of radiation of the unnecessary electromagnetic waves, it was found that the radiation of the unnecessary electromagnetic waves becomes remarkable and difficult to be suppressed when the following conditions coincide. That is, the conditions are (1) a case where an eigenmode (resonance) of a cavity formed inside a metal casing of the optical transceiver module exists near the frequencies of 10 GHz and 20 GHz and, (2) a case where an IC serving as an excitation source and wiring connected thereto (transmission line and the like) are arranged at a position where the eigenmode is excited.
In that case, strong cavity resonance occurs inside the metal casing, and the radiation of the unnecessary electromagnetic waves extremely increases. According to our examination, when the inside of the metal casing is of a rectangular solid shape in the XFP MSA optical transceiver module, a cavity resonant frequency is defined by the dimensions of the optical transceiver module, and the cavity resonant frequency occurs near 10 GHz. In addition, the cavity resonance intends to generate surface current in a perpendicular direction of inner side surfaces of the metal casing. The opening of the metal casing provided to the edge connector of the printed circuit board is orthogonal to this current, and hence the opening functions as a good slot antenna. When the cavity resonance occurs due to the above-mentioned operations, a shielding effect on the unnecessary electromagnetic waves generated from the printed circuit board by using the metal casing becomes remarkably small.
In the technologies described in the Non-patent Document and JP 2006-84683 A, the reduction effect cannot be attained regarding the radiation of the unnecessary electromagnetic waves from the opening of the metal casing of the edge connector. Further, in the technologies described in U.S. Pat. No. 6,999,323 B1 and U.S. Pat. No. 7,068,522 B2, when the cavity resonance described above occurs, the reduction effect is insufficient.
As another method of reducing the radiation of the unnecessary electromagnetic waves due to the cavity resonance, there may be adopted a method of causing a loss of the cavity resonance so as to lower a Q value and a method of changing the resonant frequency so as to avoid the cavity resonance. As specific means for the former, there is known means for arranging an electromagnetic wave absorber formed of a magnetic material and the like inside the metal casing.
However, while the electromagnetic wave absorber is highly effective at low frequency, a permeability of the magnetic material becomes remarkably low at high frequency such as 10 GHz or 20 GHz. Therefore, sufficient reduction effect is difficult to be attained. Further, the electromagnetic wave absorber is a relatively expensive component, and hence there arises a problem in that cost of the optical transceiver module increases when using a large number of the electromagnetic wave absorbers. As the latter means, there may be adopted a method of reducing the dimensions of the inside of the metal casing to raise the resonant frequency. However, it is necessary to largely reduce the area of the printed circuit board capable of being built-in, whereby it is difficult to mount components such as the IC.